Supraventricular pacemaker underdrive in the absence of sinus nodal influences in the conscious dog

The ability to reduce and maintain heart rate by electrical stimulation (underdrive) was tested in three groups of chronically instrumented dogs: sinoatrial node intact, ectopic atrial pacemaker produced by injection of rapidly hardening latex into the sinoatrial nodal artery, and idioventricular pacemaker produced by injection of formalin into the atrioventricular node. In the conscious unsedated state, underdrive of sinoatrial or idioventricular pacemakers resulted in competition between driven and intrinsic foci. However, the cycle length of ectopic atrial pacemakers could be increased by 148.4 +/- 30.7 msec (P less than 0.001) and maintained at that value. Cessation of underdrive resulted in a period of pacemaker suppression similar to that produced following overdrive. Single premature stimuli produced marked cycle length prolongations in dogs with ectopic atrial foci. Total autonomic blockade had no significant effect on the production of underdrive. Thus, the results of the present experiments provide evidence for maintained capture of cardiac pacemakers at rates significantly below intrinsic control and indicate that underdrive varies as a function of pacemaker site. Underdrive may explain the failure of junctional escape in the presence of slower atrial rhythms.